1. Field of the Invention
Aspects of the present invention relate to a process for producing a semiconductive porcelain composition/electrode assembly having a positive resistance temperature, which is used in a PTC thermistor, a PTC heater, a PTC switch, a temperature detector and the like.
2. Description of the Related Art
Compositions comprising BaTiO3 having added thereto various semiconductor dopants have conventionally been proposed as materials showing a PTCR characteristics (Positive Temperature Coefficient of Resistivity). Those compositions have a Curie temperature around 120° C. It is required for those compositions to shift the Curie temperature, depending on the use.
For example, it is proposed to shift the Curie temperature by adding SrTiO3 to BaTiO3. In this case, however, the Curie temperature shifts to only a negative direction, and does not shift to a positive direction. Currently, only PbTiO3 has been known as an additive element for shifting the Curie temperature to a positive direction. However, PbTiO3 contains an element that causes environmental pollution. Therefore, a material free of use of PbTiO3 has been demanded in recent years.
Great characteristic of the PTC materials is that the resistivity of a PTC material suddenly rises at a Curie point (jump characteristic=temperature coefficient of resistance α). This is considered for the reason that resistance (resistance by Schottky barrier) formed in a crystal grain boundary is increased. PTC materials are required to have high jump characteristic of the resistivity as their properties.
The Pb-free PTC materials as described in Patent Document 1 have such tendency that the materials having excellent jump characteristic have high room temperature resistivity, and the materials having poor jump characteristic have too low room temperature resistivity, so that there was a problem that the materials cannot satisfy both stable room temperature resistivity and excellent jump characteristic.
To overcome the above and/or other problems of the conventional BaTiO3 semiconductor ceramic, the present inventors previously proposed a semiconductive porcelain composition in which a part of Ba of BaTiO3 is substituted with Bi—Na and which have a P-type semiconductive component at a crystal grain boundary, which is a semiconductive porcelain composition having a composition formula represented by [(BiNa)x(Ba1-yRy)1-x]TiO3 (R is at least one of rare earth elements) in which x and y satisfy 0<x≦0.3 and 0<y≦0.02 or a semiconductive porcelain composition having a composition formula represented by [(BiNa)xBa1-x][Ti1-zMz]O3 (M is at least one of Nb and Sb) in which x and z satisfy 0<x≦0.3 and 0<z≦0.005, as a material in which a part of BaTiO3 is substituted with Bi—Na and which can shift a Curie temperature to a positive direction and shows excellent jump characteristic while greatly decreasing room temperature resistivity, without using Pb (Patent Document 2).
These semiconductive porcelain compositions shift a Curie temperature to a positive direction without using Pb and show excellent jump characteristic while greatly decreasing room temperature resistivity, but had a problem of change with the passage of time such that when used as a heater material, electric resistivity of the material changes. It has been known that when oxygen content in these BaTiO3 semiconductive porcelain compositions changes, a carrier concentration is changed, whereby electric resistivity is changed. Because the above-mentioned materials are sintered in an inert gas atmosphere, the materials are sintered in a state of deficient oxygen. Therefore, when the materials are used in the air, deficient amount of oxygen changes, and resistivity is liable to change. In particular, when energization is conducted in a state that an electrode is joined as a heater material, transfer of oxygen occurs among oxygen in the air and between a semiconductive porcelain composition and an electrode when the material has high temperature of from 50° C. to 120° C., whereby room temperature resistivity is changed.
Patent Document 1: JP-A-56-169301
Patent Document 2: Japanese Patent Application No. 2007-333528